This invention relates generally to a motor vehicle equipped with a diesel propulsion engine including an exhaust system having a discontinuously regenerating exhaust gas purification system with a catalytic converter unit that burns diesel fuel catalytically. The continuously regenerating exhaust gas purification system may include diesel particulate filters and NOx, accumulating catalytic converters.
To comply with environmental specifications, the exhaust gases of motor vehicles propelled by combustion engines are purified. Specifically, appropriate particulate filters are utilized to reduce the particulate emission of the exhaust gases of motor vehicles propelled by diesel engines. The particulate filters need to be periodically regenerated by burning off the particles accumulated on the filter surface. Due to the relatively low exhaust gas temperatures of modem diesel engines, to initiate regeneration, the exhaust gas temperature must be raised using appropriate measures or devices if filter regeneration is to be feasible in conditions other than full load operation, even when using fuel additives capable of lowering the particulate ignition temperature. Without the additive's, and depending on the soot composition, the particulate ignition temperature ranges between 470° C. and 600° C., by approximately 100° C., but in the long run cause the particulate filter to clog, necessitating cleaning.
For this purposel various burners (refer to DE 19504183 A1 and DE 19717544 A1) have been installed upstream of the particulate filter. However, there are several drawbacks to employing burners. For one, they are expensive. Additionally, an increased amount space is required for installation due to the integration of the comparatively large burner into the exhaust system. Finally, the burner has a detrimental effect on the flow conditions within the exhaust pipe.
One solution proposed in EP 132166 A1 is using a low-boiling, organic fluid that evaporates on a glow plug that extends into the exhaust gas carrying component connected upstream of the particulate filter where the vapors ignite either after mixing with the oxidic exhaust gas at the tip of the glow plug or, without ignition, are mixed with the stream of exhaust gases. In the latter case, catalytic oxidation of the vapors of the low boiling organic fluid in a catalytically coated area of the particulate filter causes a temperature increase of the exhaust gases ducted to the particulate filter. A particular disadvantage of this is the engineering and logistic effort concerning the storage of the low boiling organic fluid. Furthermore, the regeneration of the diesel particulate filter utilizing this heating device requires exhaust gas temperatures above 450° C., which in modern diesel engines can only be attained under full load.
DE 3139565 A1, from which a general motor vehicle may be derived, describes the injection of diesel fuel using atomizer nozzles in, or immediately ahead of, a catalytically coated area of a particulate filter to increase the exhaust gas temperature by catalytic oxidation of the fuel. An electrical heating element imbedded in the catalytically coated area of the particulate filter initiates catalytic oxidation. There are several disadvantages of this system. For one, there is considerable non-homogeneity of the temperature distribution. Additionally, the atomized fuel absorbs heat from the exhaust gas stream through partial evaporation. Additionally, given the conditions prevailing in non-purified exhaust gases the atomizer nozzles tend to clog, rendering the affected heating device non-functional within a short period of time and explaining why these systems never went into production.
Finally, attempts have been made to increase the exhaust gas temperature to the ignition temperature of the soot collected on the particulate filter by using electrical heating elements. These attempts were quickly aborted because the required electrical power was not able to be provided in conventional motor vehicles.
NOx accumulating catalytic converters are increasingly used to reduce the emission of nitrogen oxides. Regeneration of the former requires an enrichment of the exhaust gases. While this is not a problem with gasoline engines, diesel engines operating with excess air and whose exhaust gases also contain excess air require special measures to be taken.